The invention concerns a hydrodynamic retarder for a vehicle, such as an automobile. The hydrodynamic retarder is of the type having a primary part having an input end which can be connected to the transmission output shaft of the vehicle. The output end can be connected to the axle drive shaft of the vehicle. The retarder has a fixed secondary part which, relative to the primary part, is sealed against fluid discharge. The primary and secondary parts are under formation of a working space provided with feed lines and drain lines while forming a working circuit in which a working medium flows. A cooling circuit carries the cooling volume flow, with a heat exchanger incorporated into it and, as the case may be, with additional cooling and energy storing capacities, moreover with a control valve for controlling the volume flow in the outlet line.
In particular, the invention concerns the design of the control and regulating part of such a retarder in view of the desired braking moment.
DE 35 45 658 describes a hydrodynamic retarder of the general type described in the first paragraph above. Concerned here is the design of the retarder as a maximally compact, self-supporting unit. Incorporated in the inlet and outlet of the working circuit is a control valve each by which the filling and emptying of the working space, and thus the braking moment, are controlled. DE 21 50 115 concerns a control system for a hydrodynamic retarder. Provided in the cooling circuit is a switching valve for interruption of the cooling circuit and emptying of the working space as the brake is deactivated. To achieve short response times of the brake in filling and emptying, along with a high braking output, this document provides for a hydraulic receiver which is followed by a charging valve.
Underlying DE 27 06 950 is the problem of providing a retarder where the progression of the brake moments curve can be adjusted with maximally simple means. To that end it is proposed to provide the cooling circuit line with an injector type constriction at the emptying point of the replenishing line.
DE 28 55 654 is based on a retarder whose degree of filling is adjustable for adjustment of the braking moment, according to a variable set value by means of an overflow valve arranged in the outlet line, with the movable valve element of this overflow valve being adjustable by a pressure force generated by the fluid pressure in the outlet line. To accurately adjust the hydrodynamic braking moment to a set value it is proposed to control the magnitude of the opposing force by means of a controller.
DE 35 11 795 concerns as well a retarder where point a, the braking force, is effective without interruption down to low speeds. This retarder operates with two independent working circuits, which involves a high design expense.
DE 31 13 408 also concerns a hydrodynamic retarder, where the brake moments curve is to be achieved by radial, relative displacement of rotor and stator.
Most of these prior retarders have the disadvantage of high design expense. With most retarders, furthermore, the outlet choke is exclusively geared to achieving a specific operating point in the characteristic diagram of the retarder. The outlet choke with its rigidly fixed cross section assumes a relatively high share of the entire flow resistance. The remaining flow resistance is formed of individual resistances, composed of the resistances of the lines carrying the coolant, the resistance of one or several heat exchangers and additional cooling or energy absorption capacities.
Experience shows that the braking values at operating start are distinctly lower than after a certain period of operation. This effect is enhanced or prolonged by additional cooling or energy capacities. Therefore, the integration of such a retarder in the operational brake system, for instance in the mechanical friction brake of a modern truck equipped with antilock brakes, is not possible. Furthermore, the highest output valve is achieved in the operating point, though, but not the maximally possible volume flow, since due to the fixed adjustment of the outlet choke and due to the control to a constant braking moment the pumping effect of the retarder rotor is reduced as well at increasing speed of rotation. This is attributable to the reduction of the circuit charge.
Therefore, the problem underlying the invention is to design a retarder in such a way that a desired braking moment is cleanly adjustable at any desired point in time, i.e., also at the start of the braking operation, and that the working mapping of the retarder can be configured independently of the point in time of operational start.